Quantification of Fe-oxidation state in mixed valence minerals: a geochemical application of EELS revisited

Abstract

The use of TEM and electron energy loss spectroscopy to determine levels of Fe3+ in solids at nanometre spatial resolution is well established. Here we assess this technique at the energy resolution of a monochromated electron source (0.3 eV) and with the capability of simultaneously acquiring core- and low-loss spectra for absolute energy calibration of core-loss chemical shifts. Fe L2,3-edges of four oxide standards give a characteristic energy loss near edge structure (ELNES) and the L3-peak maxima of the spectra exhibit an energy shift of ∼ 1.8 eV between ferrous (709 eV) and ferric (710.8 eV) iron. We show that diagenetic chlorites present in iron oxide loaded ‘red beds’ contain predominately Fe3+, based on the absolute energy position of the low signal Fe L3-edges (centred around 710.8 eV). We also fit our standard-spectra to Fe L2,3-edges from magnetite and maghemite nanoparticles to investigate iron site occupancy. An additional chemical shift needs to be applied to the octahedral ferrous iron site component to achieve a plausible, linear fit to the magnetite spectrum which we speculate may be due to electron hopping between the mixed valence octahedral iron sites in magnetite.